Shanghai has a subtropical maritime monsoon climate. The four seasons are quite distinct, while Spring and Autumn are a little shorter than Summer and Winter.
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Due to the fact that Shanghai is right next to the sea and Yangtze River, it is quite humid in Shanghai. Although the absolute maximum temperature range does not look crazy, humidity can make the Summer feel quite hot and the Winter suffering.
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In July or August, which are the hottest time of Shanghai, the highest temperature is usually between 30 °C (86 °F) and 38 °C (100 °F). Sometimes it could go as high as 30 °C ( 104°F), but it can only be a few days at the most. The coldest time is December and January. It usually is between -2 °C ( 28°F) and 5 °C ( 41°F). Still, humidity makes it actually feel colder than the number looks.
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The recorded highest temperature that happened in Shanghai was 39.6 °C ( 103.3°F), and -8 °C ( 17.6°F) for lowest.
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It rains quite often in Shanghai. You should expect raining at least once or twice in a week. There is a raining season in June. Raining could last for several days or even weeks in this season.
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Shanghai has a fairly low crime rate. So generally, travellers do not need to worry about their own safety much in Shanghai. However, due to the fact that there are always large crowds almost everywhere in the city, it can never be wrong to keep wallets and other value stuff safe. A bag at your back is not really an absolutely safe place. There are some teenage thieves who always aim at the bags.

When running into trouble, don't panic. Shanghai Police are quite responsible and can respond very quickly. Dial 110 for Police, 120 for ambulance and 119 for fire emergency. And it is a good idea to call the consulates of your country in Shanghai.